Android: Setting width percentage of view in RelativeLayout

I'm using a RelativeLayout, and want to center the orange bar below in the center and fill up 90% of the width. I know this is possible to do using a LinearLayout using percentage, but how do I do so with a RelativeLayout?

Do I have to programatically calculate how wide is the screen (in dp), and set this 90% of this as the width of the orange view?

In short, I want something like this, where the orange bar takes up 90% of the screen centered in the middle using a RelativeLayout, without hardcoding x and y coordinates, so that it works on all screen densities. (The dark bars in the edge are the edge of the phone)